1. Field of the Disclosure
The disclosure relates to a method of measuring the effective directivity (residual system directivity) and/or the effective source port match (residual system port impedance match) of a test port of a system-calibrated vector network analyser.
2. Related Technology
The great accuracy of vector network analysers (VNA) is based on the fact that, before the actual measurement of value and phase of the complex reflection coefficient, the network analyser is calibrated at its test ports by connecting calibration standards. Meanwhile there is a large number of different calibration methods. For system calibration, the most common of these use open-circuit, short-circuit and match calibration standards. By connecting these calibration standards to the test ports of the network analyser, it is possible to determine the errors occurring in the network analyser which lead to a deviation of the measured values from the true value, and this information may then be used in the subsequent object measurement for error correction by calculation. This is known for example from DE 39 12 795 A1. However, these calibration methods as commonly used to date are not sufficiently accurate.
In order to determine the still remaining residual uncertainty of the directivity and/or test port match, it is proposed in an EA guideline that a falsely-terminated or short-circuited precision coaxial air line, defined at the outlet, be connected to the test port to be measured of the previously system-calibrated network analyser, and that the reflection coefficients be measured at the inlet to this air line, at a sequence of measuring points within a predefined frequency range of the network analyser (EA-10/12, EA Guidelines on the Evaluation of Vector Network Analysers (VNA), European Co-operation for Accreditation, May 2000). According to this guideline, though, only the so-called ripple amplitude of the oscillation overlying the value of the reflection coefficients is evaluated, and it is assumed as a simplification that this ripple amplitude is broadly identical to the effective source port match, which however is true only if the effective directivity is ignored. This known verification standard using a precision air line is therefore relatively inaccurate and allows no precise estimate of the measuring uncertainty to be expected, let alone any subsequent correction of the error correction terms for the source port match.